The adult human eye is a slightly asymmetrical sphere with an approximate sagittal diameter of 24 to 25 mm, a transverse diameter of 24 mm, and a volume of about 6.5 cc. The human eye can be divided into three different layers namely, an external layer, an intermediate layer and an internal layer. The external layer of the eye consists of the sclera, which is often referred to as the “white of the eye,” and the cornea, which covers the front of the eye. The intermediate layer is divided into an anterior portion and a posterior portion; the anterior portion consists of the circular pigmented iris, the crystalline lens and ciliary body, while the posterior portion consists of the choroid layer. The internal layer consists of the retina, which is the sensory part of the eye. The retina is essentially a layer of nervous tissue, which runs along the inside rear surface of the choroid layer and can be divided into an optic portion and a non-optic portion. The optic portion, which participates in the visual mechanism, contains the rods and cones that are the effectual organs of vision.
The human eye can also be divided into three chambers. The anterior chamber between the cornea and the iris, and the posterior chamber between the iris and the crystalline lens, are filled with aqueous humor. In contrast, the vitreous chamber between the crystalline lens and the retina is filled with a more viscous liquid, called the vitreous (also known as the vitreous body or vitreous humor). The vitreous humor in a normal eye is a clear gel occupying about 80% of the volume of the eyeball. Light that enters the eye through the cornea, pupil, and lens, is transmitted through the vitreous to the retina.
The vitreous humor of a normal human eye is a gel that is roughly 99% water and 1% macromolecules. These macromolecules include a network of collagen fibrils, hyaluronic acid, soluble glycoproteins, sugars and other low molecular weight metabolites. Type II collagen is the principal fibrillar collagen of the vitreous, but the vitreous also contains collagen types V, IX, and XI. The posterior portion of the vitreous body, the posterior hyaloid surface (also known as the posterior vitreous cortex), is in direct contact with the inner retinal surface most prominently at the vitreous base, optic disc, and along the major retinal vessels. Normal adhesion of the vitreous to the retina is mediated by cellular and molecular interactions between the posterior vitreous cortex and the inner limiting membrane (ILM) of the retina. The ILM is essentially the basement membrane of retinal Mueller cells. The ILM contains collagen types I and IV, glycoproteins such as laminin and fibronectin and other glycoconjugates. These components are thought to bridge and bind collagen fibers between the vitreous and the ILM.
With age, the vitreous humor changes from gel to liquid and as it does so it gradually shrinks and separates from the ILM of the retina. This process is known as “posterior vitreous detachment” (PVD) and is a normal occurrence after age 40. However, degenerative changes in the vitreous may also be induced by pathological conditions such as diabetes, Eale's disease and uveitis. Also, PVD may occur earlier than normal in nearsighted people and in those who have had cataract surgery. Usually, the vitreous makes a clean break from the retina. Occasionally, however, the vitreous adheres tightly to the retina in certain places. These small foci of resisting, abnormally firm attachments of the vitreous can transmit great tractional forces from the vitreous to the retina at the attachment site. This persistent tugging by the vitreous often results in horseshoe-shaped tears in the retina. Unless the retinal tears are repaired, vitreous fluid can seep through this tear into or underneath the retina and cause a retinal detachment, a very serious, sight-threatening condition. In addition, persistent attachment between the vitreous and the ILM can result in bleeding from rupture of blood vessels, which results in the clouding and opacification of the vitreous.
The development of an incomplete PVD has an impact on many vitreoretinal diseases including vitreomacular traction syndrome, vitreous hemorrhage, macular holes, macular edema, diabetic retinopathy, diabetic maculopathy and retinal detachment. Thus, an important goal of vitreous surgery is to separate the vitreous from the retina in a manner that prevents vitreous traction.
In order to remove the vitreous from the eye, a microsurgical procedure called vitrectomy is usually performed. In this procedure the vitreous is removed from the eye with a miniature handheld cutting device while simultaneously replacing the removed vitreous with saline solution to prevent collapse of the eye. Surgical removal of the vitreous using this method is highly skill-dependent, and complete removal of the cortical vitreous remains a difficult task. Furthermore, mechanical vitrectomy carries the risk of complications such as scarring, tearing and other damage to the retina. Obviously, such damage is highly undesirable as it can compromise the patient's vision after surgery.
Thus, alternative methods to remove the vitreous from the retina have been the focus of recent investigation. Such methods have explored the use of enzymes and chemical substances, which can be used to induce/promote liquefaction of the vitreous and/or separation of the vitreoretinal interface (PVD). These approaches, which are referred to as “pharmacological vitrectomy,” have included several proteolytic enzymes such as alpha-chymotrypsin, hyaluronidase, bacterial collagenase; chondroitinase and dispase, which have been injected intravitreally in experimental and/or clinical trials to induce PVD. However, most of these techniques do not release the posterior hyaloid from the ILM completely or without complications. In addition, in several of these cases, the risk of adverse reactions is high. For example, the use of bacterial proteases in mammalian systems generates an immune response, which leads to proliferative vitreoretinopathy resulting in complex retinal re-detachment. Collagenase has been reported to liquefy the vitreous, but it has also been shown to disrupt the outer layers of the retina. Alpha-chymotrypsin has been reported to produce peripapillary and vitreous hemorrhage in the injected eyes. Finally, dispase has been reported to cause toxicity to the inner layer of the retina 15 minutes after injection. Depending on the concentration of dispase used, proliferative retinopathy or epiretinal cellular membranes can develop in the injected eyes.
Given the immunogenicity and other adverse effects of bacterial proteases, pharmacological vitreolysis using endogenous human derived proteases may be desirable. Plasmin is a serine protease derived from plasminogen. Plasminogen is an important component of mammalian blood. Human plasminogen is a single chain glycoprotein consisting of 791 amino acids, which has a molecular weight of about 92,000 daltons (see Forsgren M. et al., FEBS Lett. 213(2):254-60, 1987). Native plasminogen with an amino-terminal glutamic acid (termed “Glu-plasminogen”) is converted by limited digestion by plasmin of the Arg68-Met69, Lys77-Lys78, or Lys78-Val79 peptide bonds to proteins commonly designated as “Lys-plasminogen.” Activation of plasminogen by plasminogen activators such as urokinase or streptokinase, cleaves the peptide bond between Arg561 and Val562 converting the plasminogen molecule into a double chain, enzymatically active form called plasmin. Plasmin contains two polypeptides, a heavy A chain connected by two disulphide bonds to a light B chain; the B chain contains the serine protease catalytic domain. The serine protease catalytic activity of plasmin has been implicated in its ability to dissolve blood clots in vivo.
Recently, plasmin has also been suggested as an adjunct for vitrectomy. In addition, autologous plasmin enzyme (APE) has been suggested as an agent for pharmacological vitrectomy. However, there are several disadvantages associated with the use of plasmin. First, so far all clinical interventions with plasmin have relied on the use of APE, the isolation of which necessitates a laborious and time-consuming process involving drawing of a patient's blood, isolation of plasminogen, activation of the isolated plasminogen to plasmin, and purification and sterility testing of the plasmin enzyme. Furthermore, this procedure can be costly and the presence of blood-borne pathogens can further complicate this procedure. Also, plasmin is highly prone to degradation and thus cannot be stored for prolonged periods prior to its use. A further disadvantage is plasmin's large molecular weight, which ranges between 65,000 and 83,000 daltons. Thus, the diffusion of large molecules like plasmin from its injected position in the vitreous to the vitreoretinal interface would be hindered compared to smaller molecules.
Accordingly, there is a need in the art for methods of treating or preventing disorders, or complications of disorders, of the eye of a subject that overcome the disadvantages of plasmin, for pharmacological vitreolysis. Specifically, there is a need for methods of treating or preventing a disorder, or a complication of a disorder, of the eye using smaller molecules than plasmin, which can diffuse through the vitreous to the vitreoretinal interface faster than plasmin, and which can be readily obtained in large quantities without the delay and other attendant problems of isolating autologous plasmin enzyme on a patient-by-patient basis.